A measurement of the W boson helicity is presented, where the W boson originates from the decay of a top quark produced in pp collisions. The event selection, optimized for reconstructing a single top quark in the final state, requires exactly one isolated lepton (muon or electron) and exactly two jets, one of which is likely to originate from the hadronization of a bottom quark. The analysis is performed using data recorded at a center-of-mass energy of 8 TeV with the CMS detector at the CERN LHC in 2012. The data sample corresponds to an integrated luminosity of 19.7 fb −1 . The measured helicity fractions are F L = 0 . 298 ± 0 . 028 (stat) ± 0 . 032(syst), F 0 = 0 . 720 ± 0 . 039 (stat) ± 0 . 037(syst), and F R = −0 . 018 ± 0 . 019 (stat) ± 0 . 011(syst). These results are used to set limits on the real part of the tWb anomalous couplings, g L and g R .